The ability to perform hard physical work was studied quantitatively in 12 well-motivated men who undertook to step up and down a 22-inch bench 30 times every minute until exhausted. By this procedure a 150–lb. man performs external work at a rate of 0.25 h.p. or 1140 kg-m/min. When first tested, nonathletes were not able to step as long as athletes, inactive subjects as long as active ones, or athletes out of training as long as those in full training. Seven subjects exercised repeatedly. All found that their endurance increased by 100% or more. In five of them the improvement exceeded 500%. Reductions in pulse rate, at rest and after standard periods of stepping, accompanied the training process. Submitted on December 7, 1959